1. Field of the Invention
The present invention relates generally to a CDMA (Code Division Multiple Access) communication system, and in particular, to an apparatus and method for forward power controlling in a receiving device.
2. Description of the Related Art
Power control in a CDMA mobile communication system is described in two ways: forward power control and reverse power control. The forward power control is performed on the transmission power of a base station so that a mobile station can receive a forward traffic channel with a predetermined SIR (Signal to Interference Ratio) from the base station. By the forward power control, the base station increases its transmission power for a mobile station in a bad channel environment and decreases it for a mobile station in a favorable channel environment so that the mobile stations can demodulate data stably. In terms of a system, the forward power control prevents power consumption of a traffic channel transmitted to mobile stations, thereby increasing the capacity of each cell.
Meanwhile, a mobile station is highly susceptible to interference from base stations other than a serving base station when the mobile station is at the boundary of a cell or behind a building. This is termed a corner problem of a forward channel. In this case, the mobile station needs higher reception power than a mobile station near to the serving base station. Therefore, the mobile station notifies the base station of channel status according to the error rate of received frames and the serving base station controls its transmission power based on the received channel status information to allow the mobile station to maintain a predetermined reception power level. This forward power control can be closed-loop power control.
The channel status is checked by measuring the SIR of a signal received at the mobile station. The mobile station can also compare the SIR measurement with a threshold variable depending on the error rate of frames in a demodulator and notify the base station of the comparison result. The threshold may vary in proportion to the frame error rate. The forward power control using the variable threshold corresponding to the frame error rate is called an outer-loop power control.
Meanwhile, according to the radio standard of CDMA-2000, channel status is determined by measuring the ratio of the reception power of a forward channel to interference from other cells, that is, the SIR of a received signal, for forward power control. In this method, a rake receiver in each mobile station measures interference from each path and the strength of a user channel input signal from each path, and compares an SIR with a predetermined threshold.
The interference can be measured using a pilot channel or a sync channel. As an alternative, an approximate interference strength can be measured utilizing orthogonal noise components.
As suggested currently, the reception strength of a forward user channel is measured by measuring the size of the symbols of a power control command directed to a mobile station by a base station for reverse closed-loop power control. In prior art, a mobile station detects the position and size of a power control command symbol in each power control group of a received forward traffic channel signal and squares the symbol size value. However, only a part of a power control group is used for measuring the user channel strength in the conventional technology. Consequently, the forward channel measurement is inaccurate due to lack of symbols for use in the forward user channel measurement. For example, if only three or four power control command symbols among 36 symbols of a 1.25 msec power control group are used in measuring the reception strength of a forward user channel, the measurement is inaccurate in a bad channel environment such as a fading channel. As a result, the mobile station transmits a wrong forward power control command to the base station and then the base station cannot perform a desired forward power control. That is, the base station may increase its high transmission power or decreases its low transmission power based on the wrong forward power control command, resulting in inefficient power use. Traditionally, use of unnecessary high transmission power decreases the entire available cell capacity.
FIG. 1 is a block diagram of a device for measuring the SIR of a forward user channel using reverse power control symbols in a conventional CDMA mobile communication system.
A finger 100 subjects an input signal to PN despreading and orthogonal despreading and outputs the despread, demodulated symbols to a combiner. To demodulate symbols of signals received from different paths, a plurality of fingers 100 are provided. The combiner combines the outputs of the fingers corresponding to the multiple paths and outputs the combined value to a channel decoder.
A despreader 110 in the finger 100 despreads input I- and Q-channels by multiplying them by a PN sequence received from a PN generator 160. A multiplier 115 extracts orthogonally demodulated traffic channel data by multiplying the output of the despreader 110 by a traffic channel orthogonal code Wt received from an orthogonal code generator (not shown). A symbol accumulator 120 accumulates the output of the multiplier 115 in symbol units.
A multiplier 140 extracts pilot channel data by multiplying the output of the despreader 110 by a pilot channel orthogonal code Wo received from the orthogonal code generator. A symbol accumulator 145 accumulates the output of the multiplier 140 in symbol units.
A multiplier 125 multiplies the output of the symbol accumulator 120 by a complex conjugate of the output of the symbol accumulator 145. That is, the multiplier 125 multiplies the traffic channel data by the complex conjugate of the strength and phase component of a pilot channel, thereby compensating for the phase component of the traffic channel data. The multiplier 125 outputs the demodulated symbols to the combiner (not shown).
Meanwhile, a power control symbol squarer 130 measures the power levels of reverse power control command symbols inserted in predetermined positions of the orthogonally demodulated channel data received from the symbol accumulator 120. A symbol accumulator 135 accumulates the output of the power control symbol squarer 130 in symbol units and outputs the strength of the received forward user channel. Here, since two symbols are inserted in each power control group, a frame having a total of 32 reverse power control command symbols can be defined as a corresponding accumulation period.
An interference signal measurer 150 measures the interference strength of the output of the despreader 110. A divider 155 calculates an SIR by dividing the received forward user channel strength by the interference strength and outputs the SIR to a forward power controller (not shown). The forward power controller sums the SIRs received from the plurality of fingers and determines a forward power control command to be transmitted to the base station according to the frame error rate of the channel decoder (not shown).
In the conventional method, a forward user channel reception strength is obtained by squaring reverse power control command symbols transmitted in each power control group of the forward traffic channel signal.
To solve problems of the conventional method that utilizes only a part of a power control group for measuring the strength of a received forward user channel, the present invention suggests a method for measuring the strength of a received forward user channel that utilizes all of the symbols of the forward user channel. The symbol rate of a frame in received user data changes every 20 ms according to the voice activity of a user. That is, a transmission power level per symbol of each data frame varies depending on symbol repetition at a transmitter. For example, a transmission power per symbol at a frame transmission rate of 4800 bps is a half that at 9600 bps. A transmission power per symbol at 2400 bps is a fourth that at 9600 bps. Hence, SIRs being a basis of channel measurement cannot be compared with a threshold produced without considering the symbol rate of each received frame.
Although the symbol rate of a received frame can be detected by channel decoding following symbol demodulation, a fading channel environment of a received signal has already changed at the time point of the symbol rate detection. Therefore, the channel measurement is impossible. Only if information about the symbol rate of a frame is written in a data frame format and a power measuring device at a receiver can accurately analyze the information, a forward user channel can be accurately measured using all symbols of a received frame. Here, the power measuring device considers a transmission power per symbol variable with the symbol rate of a received frame by comparing the SIR of each received frame with a threshold that varies depending on the symbol rate information.
It is, therefore, an object of the present invention to provide a data frame structure having symbol rate information to accurately measure the strength of a received forward user channel signal in a CDMA mobile communication system.
It is another object of the present invention to provide an apparatus and method for forward power controlling by measuring the SIR of a forward user channel using all symbols of received frames in a CDMA mobile communication system.
It is a further object of the present invention to provide an apparatus and method for accurate forward power controlling by measuring the SIR of a forward user channel using all symbols of a received frame and comparing the measured SIR with a threshold corresponding to the symbol rate of the received frame in a CDMA mobile communication system.
These and other objects are achieve d by providing a forward power controlling apparatus in a CDMA mobile communication system as described herein. In the forward power controlling apparatus, an SIR measurer measures the SIR of each received frame on a forward user channel using all the demodulated symbols of the frame, a symbol rate detector detects symbol rate information inserted in a predetermined position of the frame, and a controller changes a threshold according to the detected symbol rate information, compares the threshold with the SIR, and generates a forward power control command.
According to another aspect of the present invention, there is provided a forward power controlling method in a CDMA mobile communication system. In the forward power controlling method, the SIR of each received frame on a forward user channel is measured using all the demodulated symbols of the frame, symbol rate information is detected from a predetermined position of the frame, a threshold is varied according to the detected symbol rate information and compared with the SIR, and a forward power control command is generated.